The border style is for overriding the browser’s default border styling, and can be left out if you already have existing border styles for your inputs. The padding-left gives a bit of spacing between the indicator and the text in the input.

It’s important to note that, for the width of the input to stay fixed even as the border is transitioned in, you need to make sure that your box-sizing is set to border-box. Paul Irish’s CSS snippet works like a charm: